[pmwiki-users] Checking is a $$var is set

[DaveG]

Is there a way to determine whether a variable passed into from an 
included page has a value? Something like
   (:include mygroup.mypage val=myval :)

and then inside mygroup.mypage:
   (:if VAL_IS_SET $$val:)

What seems to happen is that is $$val is not set, the actual string 
"$$val" remains.

  ~ ~ Dave